Kidney stones (also known as urinary tract stones, ureteral stones or ureteral calculi in medical terminology) are a significant burden on society and health care systems. Kidney stones form in the body when the amount of various minerals in urine exceeds an amount that can be eliminated (the metastable limit), and the excess minerals form a precipitate. Most kidney stones are comprised of calcium and oxalate, though uric acid, struvite, cysteine, and other stone compositions are also common.
Kidney stones typically form in the parts of the kidney known as the renal pelvis or calyces and can stay there for years. When a stone dislodges, it makes its way down the upper urinary tract towards the bladder. Stones often get stuck en route to the bladder in the ureter. One reason for this is that mechanical rubbing of the sharp stone on the ureter's mucosal lining causes an inflammatory response and swelling (or “edema”), which inhibits the stone's ability to pass. This obstruction impedes the passage of urine from the kidney to the bladder, which results in increased internal pressure in the kidney. This pressure rise causes nerve fibers in the kidney to stretch, which in turn results in the excruciating pain well known to accompany stones. Clinically, this pain is known as “renal colic” and comes in unexpected bursts lasting 2-18 hours, until the internal pressure of the kidney is reduced. As long as the stone remains in the urinary tract, a patient will be at risk for renal colic. Female patients describe stones as worse than natural childbirth, while male patients describe it as the most excruciating experience of their lives.
Pain relief from kidney stones typically occurs instantly after stone passage or removal. Waiting for kidney stones to pass, however, can be a long and painful process. Currently, three general types of kidney stone removal methods are used, all of which have at least some shortcomings.
Extracorporeal Shockwave Lithotripsy (ESWL) is a procedure in which shockwaves are transmitted through the body in the direction of a kidney stone, in an attempt to fragment the stone into smaller pieces. For the ESWL procedure, a patient lies on a special bed (which costs approximately $750,000), is given sedation anesthesia, and is bombarded with 45-90 shocks per minute over the course of 45 minutes to one hour. The shocks are so intense that they must be synchronized with the patient's heartbeat so as not to cause cardiac arrhythmias. ESWL outcomes are mixed: 33% of patients have a successful outcome and pass “sand,” 33% of patients pass several smaller stones with excruciating pain, and 33% of patients are unaffected by the treatment. Recent studies have raised concerns about potential long-term complications of ESWL, including hypertension and diabetes. Due to the uncertain outcomes, required sedation anesthesia, and potentially hazardous mechanism of the treatment, ESWL is indicated only for patients with 8-13 mm stones located in the kidney itself. Generally, stones of this size and location are asymptomatic.
Ureteroscopy (URS) is a procedure in which a urologist inserts an endoscope up the urethra, into the bladder, and up the ureter to the site of the stone. Using a laser, the urologist fragments the stone into smaller pieces and retracts the fragments with a retention member. The procedure requires general anesthesia, high skill level from the urologist, and anywhere from 20 minutes to one hour. The endoscope, laser source, and fluoroscopy require an investment of approximately $225,000 in capital equipment alone. The ureteroscopes themselves cost approximately $15,000 and can typically be used in only about 15 procedures before needing to be replaced or repaired. The typical amount of manipulation of the ureteroscope within the ureter during the procedure, as well as the overall time spent in the ureter, can induce ureteral stricture (blockages of the ureter caused by a process similar to scarring). The procedure outcome is generally highly effective, but due to the risk of complications and required general anesthesia, URS is generally recommended only for stones that are 8-15 mm in size.
Percutaneous Nephrectomy Lithotripsy (PCNL) is a surgical procedure in which a tube is inserted through the back into the kidney. Stones are removed through the tube, using lasers, graspers, and aspiration. Though PCNL is highly effective, its invasiveness renders it applicable only to stones larger than 15 mm.
As described above, the currently available procedures for kidney stone removal are generally quite invasive and require (1) at least sedation anesthesia and in many cases general anesthesia, (2) expansive, specialized capital equipment, and (3) experienced and knowledgeable urologists to perform the procedures. Furthermore, most small kidney stones ultimately pass without any intervention. Therefore, despite the incredible, debilitating pain involved in passing kidney stones naturally, that is typically the method of choice, since kidney stone removal methods have such significant drawbacks.
Thus, it would be advantageous to have additional treatment options for kidney stone removal. Ideally, these options would be less invasive, less expensive, less prone to side effects, and/or require less physician expertise to perform. It would also be ideal if some of the additional treatment options could be used, or adapted for use, in other parts of the body to remove other obstructions. At least some of these objectives will be met by the embodiments described herein.